One type of prior art navigation apparatus for a vehicle includes an odometer for detecting the distance over which a vehicle travels, a bearing sensor for detecting the direction of progress of a vehicle, and a map storage device. Such a navigation apparatus is capable of only displaying the present position of the vehicle on the map image read from the map storage device.
FIG. 8 shows the configuration of the above-described conventional navigation apparatus for a vehicle. The navigation apparatus includes a map storage device 1, an odometer 2, a bearing sensor 3, a switch and a touch panel 4, an input interface 5, an internal bus 6, a central processing unit 7, a RAM 8, a ROM 9, an output interface 10, and a CRT 11.
The map storage device 1 which stores map data may be in the form of a video disk, a video tape, CDROM or the like. Map data on a desired area in the map data stored in the map storage device is read out and displayed on the CRT 11. The odometer 2 generally generates distance data for unit distance by detecting the angular velocity of the wheel. The bearing sensor 3 generally generates bearing data by detecting the geomagnetic bearing corresponding to the direction of progress of the vehicle. The distance over which the vehicle travels after it has left the starting point and the direction in which the vehicle travels or whether the vehicle turns right or left are detected by a combination of the odometer 2 and the bearing sensor 3. The CPU calculates the present position of the vehicle on a predetermined course using the measured data, and displays the obtained data on the map. A driver inputs data such as a destination, enlargement of the map and so on through the switch and touch panel 4 when the navigation is to be started so as to input the destination, the present position of the vehicle and so on in the RAM 8. The touch panel may be provided on the screen of the CRT 11. When the driver selectively touches a desired touch area in a plurality of touch areas, a corresponding signal is generated. The inputs interface 5 is a signal converter through which the input from the sensors, the storage device and the switches are sent to the internal bus 6. The ROM 9 stores programs for inputting/outputting the various types of data through the input interface 5, the output interface 10 and the internal bus 6, as well as those for processing the data. The programs are executed by the CPU 7.
The odometer in the conventional navigation apparatus determines the angular velocity of the wheel and calculates the distance over which the vehicle travels using the measured value of the angular velocity and the radius of the wheel. However, the radius of the wheel may vary owing to the air pressure or to wear thereof. Further, the vehicle may travel at a slant, for example, when it changes traffic lanes. These factors generate errors in the distance measured, resulting in the erroneous detection of a present position M, as shown in FIGS. 9 and 10.
In order to eliminate this problem, the present applicant filed an application (Japanese patent application No. 62-44797) and proposed a navigation apparatus which is capable of preventing generation of distance measurement errors by correcting the measurement errors each time it is detected that the vehicle has turned left or right on a predetermined course to its destination. However, this navigation apparatus still suffers from a problem in that, in a case where the vehicle has to travel a long distance before it reaches an intersection where it makes a turn, it is impossible to correct the present position (travelled distance), thereby accumulating measurement errors.
An object of the present invention is to provide a navigation apparatus for a vehicle which is designed to obviate the aforementioned problems of the prior art and which is thus capable of obtaining a correct travelled distance and hence a correct present position even when the vehicle has to travel a long distance before it reaches an intersection where it makes a turn.